1. Field of the Invention
The present invention relates to a structure of a semiconductor device and, more particularly, to a structure of a power semiconductor device for use in power control and a method of manufacturing the same.
2. Description of the Background Art
FIG. 8 is a schematic sectional view of a structure of a first background art power semiconductor device. As shown in FIG. 8, the first background art power semiconductor device comprises a power element 1, lead frames 2 formed from sheet metal, a metal block 5 functioning as a heat sink for heat dissipation, and a resin package 6.
The lead frames 2 have a die bonding pad portion 3 and an inner lead portion 4. The power element 1 is bonded to the die bonding pad portion 3 with solder 9. The power element 1 is formed with electrodes (not shown) connected to the inner lead portion 4 of one of the lead frames 2 through an aluminum wire 8. The metal block 5 has a protrusion provided substantially centrally thereof and spaced a predetermined distance apart from the opposite surface of the other lead frame 2 from the power element 1 in opposed relation to the power element 1. The resin package 6 seals the power element 1, the lead frames 2 and the metal block 5 while uncovering a surface of the metal block 5 opposite from the lead frames 2. An external heat dissipator 11 is mounted to an uncovered portion of the metal block 5. A portion of the resin package 6 which lies between the protrusion of the metal block 5 and the other lead frame 2 is referred to hereinafter as a resin insulation layer 27.
In some cases, an element for forming a control circuit for the power element 1 in addition to the power element 1 is formed on the die bonding pad portion 3.
FIG. 9 is a schematic sectional view of a structure of a second background art power semiconductor device. FIG. 10 is a sectional view, on an enlarged scale, of a portion B shown in FIG. 9. Such a structure is disclosed in, for example, Japanese Patent Application Laid-Open No. 10-93015 (1998). As shown in FIGS. 9 and 10, the second background art power semiconductor device comprises the power element 1, a diode 12, a heat diffusion plate 15, the lead frame 2, an insulation layer 37, a heat sink 25, and the resin package 6.
The power element 1 is bonded to the heat diffusion plate 15 made of copper with solder 9. A surface of the heat diffusion plate 15 opposite from the power element 1 is bonded to the die bonding pad portion 3 of the lead frame 2 made of copper with solder 9. A surface of the lead frame 2 opposite from the heat diffusion plate 15 is fixed to the heat sink 25 made of copper with the insulation layer 37. In the manufacturing steps, the lead frame 2 is previously fixed to the heat sink 25, and then the heat diffusion plate 15 with the power element 1 bonded thereto is bonded to the die bonding pad portion 3 of the lead frame 2. The diode 12, which generates less heat than the power element 1, is directly bonded to the lead frame 2 with the solder 9 without using the heat diffusion plate 15 therebetween. The power element 1 is formed with electrodes (not shown) connected to the inner lead portion 4 of the lead frame 2 through the aluminum wire 8. The resin package 6 seals the power element 1, the diode 12, the lead frame 2 and the heat diffusion plate 15 while uncovering a surface of the heat sink 25 opposite from the lead frame 2.
In the first background art power semiconductor device, heat generated from the power element 1 flows through the other lead frame 2, the resin insulation layer 27 and the metal block 5 and is then dissipated from the external heat dissipator 11 to the exterior. The metal block 5 and the external heat dissipator 11 are made of a material selected from the group consisting of aluminum and copper which are about 230 W/mK and about 390 W/mK in thermal conductivity, respectively. The lead frames 2 are also formed of a metal such as copper, and accordingly have a thermal conductivity similar to those of the metal block 5 and the external heat dissipator 11. The resin which forms the resin insulation layer 27 has a thermal conductivity of 1 to 3 W/mK. In this manner, the resin insulation layer 27 has the thermal conductivity which is about 1/100 those of other materials, and is therefore a main deterrent to heat conduction.
The heat dissipation characteristic of a semiconductor device is determined by the thickness and thermal conductivity of a material through which heat passes, an area over which heat passes through a material, and the like. In the first background art power semiconductor device, the reduction in the thickness of the resin insulation layer 27 may be made to reduce a portion of poor heat conduction through which heat passes, thereby improving the heat dissipation characteristic. However, the resin insulation layer 27 must have a dielectric breakdown voltage of thousands of volts, and therefore has a thickness limit of about 0.5 mm. This gives rise to a limit to the improvement in the heat dissipation characteristic.
Further, the use of ceramic powder having a high thermal conductivity, for example, powder of aluminum nitride, silicon nitride or the like as a filler to be added to the resin which forms the resin insulation layer 27 and the increase in filling factor of the ceramic powder allow the increase in the thermal conductivity of the resin insulation layer 27 up to about 5 W/mK. However, since the resin insulation layer 27 is a part of the resin package 6, this technique results in the use of the ceramic-powder-filled resin for other parts of the resin package 6 than the resin insulation layer 27, i.e., parts which need not have such a high thermal conductivity. This causes the needless use of the costly resin, to increase the cost of the materials of the semiconductor device.
The heat generated from the power element 1 passes through the other lead frame 2 and then through the resin insulation layer 27. Unlike the metal block 5, the lead frames 2, in general, are not permitted to increase the thickness thereof in terms of processing problems, and accordingly produce a lower heat diffusion effect than the metal block 5 and the like. It has therefore been difficult to sufficiently increase the area over which heat passes through the resin insulation layer 27, which has been one of the causes of the limitations of the improvement in the heat dissipation characteristic.
In the second background art power semiconductor device, the lead frame 2 and the heat diffusion plate 15 are disposed between the power element 1 and the insulation layer 37. The heat diffusion plate 15 diffuses the heat generated from the power element 1 in a horizontal direction perpendicular to the thickness direction thereof to increase the area over which heat passes through the insulation layer 37. However, the increase in the horizontal dimension of the heat diffusion plate 15 disposed between the power element 1 and the lead frame 2 for the purpose of improving the heat dissipation characteristic presents difficulties in wiring of the aluminum wire 8 which connects the power element 1 and the lead frame 2 to each other. There is also another problem such that the increase in the thickness of the heat diffusion plate 15 elongates the wiring length of the aluminum wire 8 to increase power dissipation.
To enhance the thermal conductivity, a ceramic-material-filled resin is sometimes used only for the insulation layer 37. In other words, different resins are sometimes used for the resin package 6 and the insulation layer 37. In such a case, when the resin package 6 is formed after the lead frame 2 and the heat sink 25 are bonded together, i.e., after the insulation layer 37 is cured, peeling is prone to occur at the interface between the resin package 6 and the insulation layer 37. This decreases the dielectric breakdown voltage between the lead frame 2 and the heat sink 25.